This invention relates to an inverter system for driving a synchronous motor by a plurality of unit inverters arranged in multiplexed relationship.
With the development of high speed switching semiconductor devices, inverter systems for transforming the DC to the AC power have recently expanded in the field of their application and particularly been widely employed to control the operation of AC motors. In the control of AC motors, voltage and current waveforms applied to such motors are desirably low in ripples. To this end, several unit inverters have been arranged in multiplexed relationship by disposing transformers between the unit inverters and an associated electric motor. However where this multiplexed inverters drive the synchronous motor, the same must operate the motor at a null operating frequency developed on the AC side thereof, that is, with direct current during the start of the motor and others. This has caused a problem that the transformers are saturated. As a result, multiplexed inverter systems of the conventional construction have not been employed to operate synchronous motors.
In order to solve such a problem, there are well known multiplexed inverter systems of the type comprising a DC source, a first set of three inverters connected in parallel circuit relationship across the DC source one for each phase of the system, a second set of three inverters also connected in parallel circuit relationship across the DC source one for each phase of the system, all the inverters being identical in construction to one another, a three-phase synchronous motor including three phase windings, each of the inverters of the first set including a pair of outputs connected across an associated one of the phase windings of the synchronous motor through a secondary winding of an electrically insulated transformer connected across a pair of outputs of the associated inverters of the second set.
Upon starting the synchronous motor, the three inverters of the second set are operated only to shortcircuit the mating transformers while only the three inverters of first set are operated to supply an electric power to the synchronous motor resulting in the start of the motor. When an operating frequency provided by the AC side of the inverters of the first set rises to a predetermined frequency from its null magnitude, the inverters of the second set are initiated to perform the inverting operation to induce voltages in rectangular waveforms across the secondary windings of the associated transformer respectively. As a result, output voltages from the inverters of the first set are added to those from the associated second inverters and then applied across the mating phase windings of the synchronous motor respectively. Accordingly the inverters of the first and second sets are initiaed to be operated as a multiplexed inverter system.
Therefore conventional multiplexed inverter systems such as described above are enabled to start the synchronous motor with direct current until the synchronous motor is operated with alternating current at any frequency.
In such conventional multiplexed inverter systems, however, the synchronous motor has been required to be electrically connected to two sets of inverters through six power supply leads, two for each phase of the motor. Also the number of the components has been large. For example, even though one pair of inverters of the first and second sets for each phase of the system would be formed into a single-phase inverter composed of a pair of semiconductor inverters each including four thyristors connected into a bridge configuration connected across the DC source in the well known manner, a single one of those single phase inverter has been required to be provided for each phase of the system. Therefore conventional multiplexed inverter systems such as described above have been disadvantageous in that the resulting systems are large-sized and also expensive.
Accordingly it is an object of the present invention to eliminate the disadvantages of the prior art practice as described above by the provision of an inverter system for driving a synchronous motor, using a plurality of three-phase inverters connected into a three-phase full-wave configuration and decreasing the number of power supply leads electrically connecting the three-phase inverters to the synchronous motor to four (4) by connecting phase windings of the synchronous motor into a start configuration and causing one of the power supply leads to serve as a neutral lead the synchronous motor.